Fuel cells are well known and are commonly used to produce electrical power from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power electrical apparatus such as dispersed power generators and transportation vehicles. Low temperature proton exchange membrane (“PEM”) or phosphoric acid fuel cells (“PAFC”) utilize substrates as gas diffusion layers to support the catalyst layers. Configuration of the substrates is dependent upon the configuration of the cells. Most contemporary PEM & PAFC fuel cells have ribbed bi-polar separator plates with relatively thin flat substrates. An alternative cell design uses ribbed substrates with flat bi-polar plates. Such substrate designs are well known in the art and have benefits and drawbacks depending upon many manufacturing and operational factors.
It is known that substrates may be made by wet-laid paper making technology and by dry-laid paper or felt making technology. U.S. Pat. No. 4,851,304 that issued on Jul. 25, 1909 to Miwa et al. describes a common wet-laid process used to make a typical thin fuel cell substrate beginning with a wet-laid carbon paper. This process uses 6-12 millimeter (“mm”) chopped carbon fibers to form a paper that is treated with a variety of thermoset resins, pressed to a controlled thickness, heated in an inert atmosphere to carbonize the resin. The wet-laid process is a multiple-step, complicated process that yields a thin substrate having satisfactory porosity rigidity and flexural strength for assembly into and operation within a fuel cell. The high flexural strength is primarily attributable to the 6-12 mm length of the carbon fibers.
U.S. Pat. No. 4,426,340 that issued on Jan. 17, 1984 to Goller et al. discloses a known dry-laid process that is less complex than the wet-laid process, but that results in a substrate having a lower flexural strength because the average carbon fiber length is about 0.2 mm. This dry-laid process includes mixing milled carbon fibers that are about 0.025 mm to about 0.46 mm in length with a thermoset resin powder; depositing the mixture onto a support belt of a double belt press apparatus, and then continuously compressing, molding and heating the mixture between the support belt and a compression belt to melt and cure the thermoset resin to produce a precursor substrate. The resulting precursor substrate is then trimmed, carbonized and graphitized to form a substrate for use in a fuel cell. This form of substrate, however, suffers from a low flexural strength because of the short lengths of the carbon fibers.
Problems arise with such dry-laid substrate manufacturing processes in utilizing longer carbon fibers. It is difficult to achieve a homogenous mixture with a resin or binder that affords acceptable porosity and flexural strength within the substrate. Manufacture of the substrate by an efficient dry-laid process has been limited to substrates made from very short milled carbon fibers due to an inability of known available powder feeders to uniformly distribute long fibers. The powder feeders used in known dry-laid fuel cell substrate manufacture consist of about a 90 mm brush rotating in the bottom a semi-circular hopper that is made from perforated metal. Longer carbon fibers create fiber balls that eventually block the perforations in the perforated metal. The short fibers result in a substrate with a low flexural strength which leads to excessive scrap rate during processing of the substrate into fuel cell electrodes. Additionally, known thermoset resin powders used in dry-laid substrate manufacturing processes that utilize a double belt press apparatus have cure times that range from 3 minutes to 15 minutes at 150 degrees Celsius (“° C.”) which results in a requirement for a very costly, long double belt press apparatus.